The rate and temperature at which a polyolefin such as polypropylene, polyethylene, propylene-ethylene copolymers or mixtures thereof crystallize is an important parameter, especially with respect to melt processes such as injection molding or blow molding, where the polymer is melted and then shaped into its final form. The rate at which a polymer crystallizes determines the efficiency of the process by controlling the total time required for each cycle or the rate at which the process may be operated. For example, in conventional injection molding processes, the rate at which the process can be operated is determined, to a large degree, by the amount of time required for the polymers to crystallize or "freeze" after being molded. Thus, the speed at which a melt process can be operated may be increased by enhancing the rate at which the polymer used in the process crystallizes. In addition, enhanced crystallization typically results in improved clarity and increased stiffness.
"Polyolefin," as used herein, means thermoplastic polymers derived from simple olefins, copolymers derived from olefins and blends of such polymers and copolymers. The term "polymer," as used herein, refers generically to homopolymers and to copolymers derived from two or more monomers.
In the past, enhancement of crystallization in polymers such as polypropylene, polyethylene, propylene-ethylene copolymers and mixtures thereof has been achieved by adding an extrinsic substance which acts as a seed or nuclei on which crystal growth can be initiated. Such substances are commonly referred to as nucleation agents and may consist of inorganic substances such as talc and other silicates, precipitated or ground calcium carbonates, sodium phosphates and stearates. Organic nucleating agents include dibenzylidene sorbitols and sodium benzoate. During the melt process, these substances either do not melt at all, or melt but solidify before the polymer, thus acting as nuclei for the initiation of crystallization.
The use of conventional nucleating agents has several disadvantages. First, the efficiency of the agent depends upon its particle size and the degree of dispersion and distribution of the agent in the polymer. Inorganic nucleating agents must have an extremely small particle size and be uniformly dispersed and distributed throughout the polymer to be efficient. Moreover, the addition of any foreign substance to the polymer can affect the physical and chemical properties, such as toxicity and extractability, of any product made from the polymer. This is especially critical in the case where the polymer is used to make a product that will come in contact with food or medical product.
Thus there is a need for a practical and readily achievable method of optimally enhancing the crystallization of polymers, such as polypropylene, polyethylene, propylene-ethylene copolymers, and mixtures of the same. It is also desirable to enhance the crystallization of these materials without degrading the polymer or copolymer to any appreciable extent. Preferably, crystallization enhancement is achieved by treating or using a relatively small amount of material to facilitate processing of the polymer.
Boynton, U.S. Pat. No. 4,282,076 discloses a method of visbreaking polypropylene wherein a prodegradant is formed by activating a first portion of a polypropylene polymer, mixing the prodegradant with a second portion of propylene polymer which may contain a stabilizing amount of at least one antioxidant, wherein the second portion is at least equal in amount to the prodegradant, adding to the mixture of the prodegradant and second portion of the propylene polymer a stabilizing amount of at least one antioxidant if the second portion does not already contain a stabilizing amount of such a stabilizer, and heating the mixture to an extrusion temperature to controllably lower the molecular weight of the mixture, while substantially retaining the stabilizing effect of the antioxidant stabilizer or stabilizers. The prodegradant is produced by activating a portion of the polypropylene polymer by exposure to ionizing radiation or air oxidation. After heating to extrusion temperatures, the mixture of the first activated propylene polymer and second portion of propylene polymer is purported to be significantly reduced in molecular weight and the molecular weight distribution is narrowed.
Kohyama et al., U.S. Pat. No. 4,727,113, discloses a crystalline 1-butene comprising: (a) a crystalline 1-butene polymer containing a 1-butene component as a main component, and (b) a radical-treated crystalline olefinic polymer having (1) a boiling p-xylene insoluble content of 30% by weight at most and (2) the difference between the crystallization temperature of the radical-treated crystalline olefinic polymer and the crystallization temperature of the crystalline olefinic polymer before the radical treatment being greater than or equal to 1, and (c) the proportion of the radical-treated crystalline olefinic polymer (b) being 0.2 to 100 parts by weight of the crystalline 1-butene polymer. The radical treatment purportedly may be carried out by treating the crystalline olefinic polymer in the molten state by the action of a shearing force in the presence of a cross-linking agent and a polymerization initiator, or exposing the crystalline olefinic polymer to light irradiation or ionizing irradiation in the presence of a photo-polymerization initiator.
Kirch, German Application DE 3,415,063, discloses a process for nucleation of partially crystalline plastics by irradiation wherein neutron beams are applied. Purportedly, the neutron beams, because of their different physical mode of action, as compared to electron, X-ray, gamma or ultraviolet beams, interact primarily with hydrogen atoms which reduces the number of chain breaks. Moreover, Kirch states that treatment with electron, X-ray, gamma or ultraviolet beams cause an undesired intensive degradation which alters the properties of the starting polymers. Kirch discloses neutron emitters such as americium-241/beryllium, californium-252, spent nuclear fuel rods and neutron radiation occurring in the operation of nuclear reactors as irradiation sources. These sources are, however, from a practical standpoint, difficult to effectively access and utilize due to numerous factors.
Fisher, U.S. Pat. No. 4,628,073 discloses a soft-rubbery matrix material, and a method of producing the material, wherein the material is composed of 0.3-70 micron particles of a 50,000-300,000 molecular weight cross-linkable polymer dispersed in a fluxable elastomer where the polymer's softening point temperature exceeds the elastomer's fluxing temperature and the polymer and elastomer are combined and mixed at a temperature maintained above the fluxing temperature of the elastomer but below the softening point temperature of the polymer. When a cross-linked polymer component is desired, high-energy ionizing radiation induced cross-linking is the preferred practice.
Potts, U.S. Pat. No. 3,349,018, discloses a method for controllably degrading alpha olefin polymers such as polypropylene without the use of heat and/or mechanical shear. In the method of Potts, polypropylene is degraded by subjecting it to ionizing radiation to a total dose of between about 0.01 to about 3 megareps but below that amount which causes gelation. The results of the method of Potts are attributed to uniform treatment of every portion of the resin mass by high energy ionizing radiation and it is stated that in the process each molecule of resin is surrounded by a cloud of high energy particles so that no portion of the polymer is able to escape treatment. Additionally, in a preferred embodiment of Potts, a small amount of antioxidant, preferably about 0.01 to about 0.1 percent by weight of anti-oxidant is incorporated prior to subjecting the polymer to ionizing irradiation.
Scheve, U.S. Pat. No. 4,916,198, discloses a purportedly normally solid, high molecular weight, gel-free, amorphous to predominantly crystalline, propylene polymer characterized by high melt strength due to strain hardening believed to be caused by free-end long chain branches of the molecular chains forming the polymer. The material is characterized by a branching index preferably less than 0.9 and most preferably about 0.2-0.4. Scheve also discloses a process for making the polymer by high energy radiation of a normally solid, high molecular weight, linear polypropylene polymer in a reduced oxygen environment, maintaining the irradiated material in such environment for a specific period of time, and then deactivating free radicals in the material.
There is, however, a need for a method of optimally enhancing the crystallization of polymers, such as polypropylene, polyethylene, propylene-ethylenecopolymers, and mixtures of the same. This modification should be readily achievable and practically feasible without the introduction of a foreign substance into the polymer and without degrading the polymers to any appreciable extent.